Electromagnetic coupler circuit board

ABSTRACT

An electromagnetic (EM) coupler including a first transmission structure having a first geometry, and a second transmission structure having a second geometry and forming an EM coupler with the first transmission structure, the first and second geometries being selected to reduce sensitivity of EM coupling to relative positions of the first and second transmission structures is disclosed.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application is a continuation of U.S. patent applicationSer. No. 09/714,899, filed on Nov. 15, 2000.

FIELD OF INVENTION

[0002] This invention is related to the field of electromagneticcoupling devices for bus communication.

BACKGROUND OF THE INVENTION

[0003] Electromagnetic coupling devices enable energy to be transferredbetween components of a system via interacting electric and magneticfields. These interactions are quantified using coupling coefficients.The capacitive coupling coefficient is the ratio of the per unit lengthcoupling capacitance, C_(m), to the geometric mean of the per unitlength capacitances of the two coupled lines, C₁. Similarly, theinductive coupling coefficient is the ratio of the per unit lengthmutual inductance, L_(m), to the geometric mean of the per unit lengthinductances of the two coupled lines, L₁.

[0004]FIG. 1 shows a conventional broadside coupler, where the twobroadest faces of two adjacent printed circuit board conductor lines areelectromagnetically coupled. FIG. 2 shows an edge coupler, where thenarrow faces of two conductors on the same layer are coupled.

[0005] Conventional coupling devices suffer from deficiencies in severalareas. The coupling devices exhibit significant variations in thecapacitive coupling coefficient due to manufacturing tolerances in theline geometry and in the relative position of the two coupled lines(“x,y,z variations”). Furthermore, in common manufacturing practices,the width of conductors is subject to variations of between +/−0.5 and+/−1.0 mils, the relative alignment of conductor layers within a printedcircuit board (PCB) is subject to variations of +/−5 mils (x,y axis),the distance between conductor layers can vary by +/−2 mils (z axis),and the location of holes for guide pins is subject to +/−4 milvariations (x,y axis). Therefore, conventional couplers are toosensitive to misalignment to be used in computer systems.

[0006] The present invention addresses these and other deficiencies ofconventional couplers.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007] The present invention is illustrated by way of example and notlimitation in the figures of the accompanying drawings, in which likereferences indicate similar elements, and in which:

[0008]FIG. 1 shows a prior art broadside coupler.

[0009]FIG. 2 shows a prior art edge coupler.

[0010]FIGS. 3,4, and 5 show embodiments of a portion of a couplerincluding two conductors.

[0011]FIGS. 6A, 6B, and 7 show embodiments of multiple crossed couplersegments.

[0012]FIGS. 8 and 9 show variations in capacitive coupling coefficient.

[0013]FIGS. 10A and 10B show embodiments of a coupler.

[0014]FIGS. 11A and 11B show a digital bus communication system havingmultiple couplers.

[0015]FIGS. 12A, 12B, 12C, and 12D show embodiments of a cross-sectionof a coupler.

[0016]FIG. 13 shows another embodiment of a cross-section of a coupler.

[0017]FIG. 14 shows an orthogonal view of the cross-section shown inFIG. 13.

[0018]FIG. 15 shows an embodiment of a coupler on a motherboard and aflex circuit.

DETAILED DESCRIPTION

[0019] An electromagnetic (EM) coupler is disclosed. For one embodiment,the EM coupler includes a first transmission structure having a firstgeometry and a second transmission structure having a second geometry,which may be different than the first geometry. An EM coupling is formedbetween the first and second transmission structures. For oneembodiment, the first and second geometries are selected to reducesensitivity of EM coupling to relative positions of the first and secondtransmission structures. The EM coupler structure may be physicallyseparated into two component halves to be used in an interconnectapplication.

[0020] For one embodiment, the EM coupler provides a broadband couplingdevice that is separable, bidirectional, and provides robust performancedespite misalignment of the transmission structures. The coupler mayfurther have an impedance that is controlled over a wide frequency rangeto prevent losses from reflections. Thus, the coupler may be used totransmit and receive digital signals.

[0021] For one embodiment, the EM coupler also provides bidirectionalsignal transfer; i.e. the transmission properties of the coupler areessentially the same in the forward and reverse signal transferdirections. For one embodiment, the line impedance of the EM coupler iscompatible with the circuitry of a computer system.

[0022]FIG. 3 shows a coupler that includes an arrangement of sections oftwo conductors A and B separated by a dielectric such as air, forexample. FIG. 4 shows a top view of the sections of the conductors. Asshown in FIG. 4, conductor A is rotated by an angle 410 from the commonlongitudinal axis, while conductor B is rotated by an equal but oppositeangle 410 from the same common longitudinal axis.

[0023]FIG. 5 shows a coupler having a total capacitance that includes aparallel plate capacitance and a fringe capacitance. In overlapping area510, the capacitance contribution from the overlapping sections of theconductors is generally similar to that of a parallel plate capacitorwith parallelogram shaped plates. The capacitance between the conductorsA and B in regions 520 is a fringe capacitance. The outer bounding edges525 show the points where the added fringe capacitance between the twoconductors A and B becomes negligible, e.g. less than 0.1%, of the totalcapacitance of the coupler.

[0024] The combination of the parallel plate capacitance and the fringecapacitance provides a nearly constant coupling capacitance in the faceof deviations from a nominal position. This constant couplingcapacitance provides robust coupling even if the conductors aremisaligned. Therefore, the two conductors can be moved relative to eachother in the x and y directions, without a significant change in theirmutual capacitive coupling coefficient. This constant couplingcoefficient behavior under x, y translation holds providing that thelengths of the two conductors are such that no disturbing feature, suchas, for example, the end of either conductor or a bend in either of theconductors, falls into the overlapping area 510 or the fringe regions520 of the conductors in such a manner as to significantly perturb theparallel plate and the fringe capacitance contributions. However, if adisturbing feature is present, the coupler may still function, but thecoupling coefficient may change significantly and the performance may bedegraded.

[0025] If the vertical separation distance d between the two conductorsis increased, the contribution of the parallel plate component in theregion 510 in FIG. 5 decreases as a function of 1/d. However, the fringecapacitance in regions 520 of FIG. 5 can contribute as much as 25% ofthe total coupling capacitance between the conductors. The distancebetween surface elements of the conductors in the fringe capacitanceregions is determined by both the conductor separation distance d andthe selected angle 410. The fringe capacitance contribution changes at arate significantly less than 1/d. The rate of change in the couplingcoefficient between conductors A and B, as shown in FIG. 5, separated bya distance d and rotated by a selected angle 410, is thereforesignificantly less than the rate of change between couplers havingbroadside or edge configurations, as shown in FIGS. 1 and 2, wherenearly all of the coupling capacitance shows a 1/d dependency.

[0026] The coupling coefficient may be increased by the use of multiplecrossed coupler segments for a fixed length of coupler region as shownin FIG. 6A. Referring to FIG. 6A, a conductor A has been formed frommultiple connected segments lying in a plane, where adjacent segmentsare arranged with an alternating angular displacement about thelongitudinal axis of the conductor. A second similarly segmentedconductor B is separated from conductor A by a dielectric at somepredetermined distance, with its segments lying in a plane parallel tothat of conductor A and arranged so that the angular displacement of itssegments are in the opposite sense to the corresponding segments inconductor A, to form the zig-zag structure shown in FIG. 6A. Thestructures from conductor A and conductor B have their longitudinal axesaligned collinearly in their nominal position, as shown in FIG. 6A.(Alternatively, one conductor may have a zig-zag geometry, and the otherconductor may have a straight line geometry. This alternative embodimentis shown in FIG. 6B, which shows a coupler having one straight conductorA, and also another conductor B, which is segmented in a zig-zaggeometry.)

[0027] By providing a number of parallel plate capacitance regions 610and fringe capacitance regions 620 per unit length, the geometry shownin FIG. 6A increases the capacitive coupling coefficient availablebetween the coupled conductors A and B, while retaining the alignmentinsensitivity characteristics of the coupler shown in FIG. 5.

[0028] In addition to the capacitive coupling coefficient, the coupleralso has an inductive coupling coefficient, which is derived from themutual inductance between the conductors and the self inductance of eachconductor. The mutual inductance describes the energy that ismagnetically transferred from one conductor to the other. For example, atime-varying electric current flowing through one conductor generates atime-varying magnetic field which causes an electric current to flowthrough the other conductor. The self inductance describes the energythat is stored when an electric current flows through a conductor andgenerates a magnetic field.

[0029] The inductive coupling coefficient, which is the ratio of themutual inductance between the conductors to the geometric mean of theself inductance of each individual conductor, is also proportional tothe geometric mean distance between the conductors. The mutualinductance is proportional to the length of the coupler conductors. Thecapacitive and inductive parameters of a structure with a given geometryare determined by the material properties of the structure. Therefore,once a structure has been designed with an appropriate geometry toobtain a desired set of capacitive parameters, the inductive parametersare also determined.

[0030] The interaction of the capacitive and inductive couplingcharacteristics becomes significant, especially at higher frequencies.This interaction results in directivity for the coupler. By controllingthe length of the coupler to be a preferred fraction of a wavelength ata desired lower frequency, the relative magnitude of energy flow in theforward and reverse directions on the receiving conductor of the coupler(directivity) is determined over a preferred frequency range. Forexample 1 cm of length provides approximately 3 dB directivity over afrequency range of 400 megahertz (MHz) to 3 gigahertz (GHz).

[0031] The magnitude of the coupling coefficient for the coupler shownin FIG. 6A remains substantially unchanged over a large range ofrelative x and y displacements of the conductors A and B as long as thedistance between the adjacent edges of the two conductors is greaterthan a given distance. In the limiting case shown in FIG. 7, an increasein the coupling coefficient begins to occur when the x, y displacementbecomes sufficiently large to bring the adjacent edges 710 and 720 ofthe conductors A and B into close proximity. The range of x,ydisplacements for which the coupling coefficient remains essentiallyconstant is therefore controlled by selection of an appropriate segmentlength, such as 0.125 cm for example, and an appropriate displacementangle, such as 35 degrees, for example. Further, by selection ofappropriate values for the conductor widths, conductor separation andnumber of segments, a range of coupling coefficients may be obtained.

[0032] For example, FIG. 8 shows the computed variation in capacitivecoupling coefficient for a coupler composed of 5 mil wide conductors.The x and y dimension offsets in FIG. 8 are up to 8 mils. In this range,the variation in the capacitive coupling coefficient is less than +/−2%about the average.

[0033]FIG. 9 shows the computed variation in capacitive couplingcoefficient with a change in the separation distance between the couplerconductors in the z axis. It shows that for a +/−30% change in conductorseparation, the capacitive coupling coefficient varies by less than+/−15%. This compares with parallel plate based geometries shown inFIGS. 1 and 2 which show a +40/−30% variation over the same range ofconductor separations.

[0034] In addition to the stability of the coupling coefficients of thegeometry shown in FIG. 6A, several alternative geometries may be used inthe coupler structure. These alternative geometries may reduce far-fieldelectromagnetic radiation, increase broadband behavior of the coupler,reduce impedance discontinuities, and enable the use of alternatematerials for improved performance and flexibility.

[0035] One embodiment of an alternative geometry for the EM coupler isshown in FIG. 10A. Referring to FIG. 10A, the EM coupler includes adifferential pair of conductors 1010 and 1012. Conductor 1010 is coupledto a second conductor 1014, while conductor 1012 is coupled to a secondconductor 1016. A first reference plane 1019 is placed below the firstset of conductors 1010, 1012, to act as a return conductor for thesetransmission lines. A second reference plane 1020 is placed above thesecond set of conductors 1014 and 1016 to act as a return conductor forthe transmission lines 1014 and 1016. Ends 1010B and 1012B of the firstconductors 1010 and 1012 are terminated with matched terminationresistors 1024 and 1026. Ends 1014B and 1016B of the second set ofconductors are also terminated with matched resistors 1028 and 1030.

[0036] A differential digital signal is applied to ends 1010A and 1012Aof the first conductors, and a resulting differential coupled signal isthen observed at the set of conductor ends 1014A and 1016A. Conversely,a differential digital signal is applied to ends 1014A and 1016A of thesecond conductors, and a resulting differential coupled signal is thenobserved at the set of conductor ends 1010A and 1012A. Thus, the firstand second set of conductors are reciprocally coupled by theirelectromagnetic fields. Alignment insensitivity of the coupler aidsdifferential signaling by reducing mismatches between the coupler formedby conductors 1010 and 1014 and the coupler formed by conductors 1012and 1016.

[0037] The differential coupler shown in FIG. 10A reduces the effects ofradiation. The use of differential signaling, with anti-phased currentsflowing in the differential conductor pair, causes the radiation to fallrapidly to zero as the distance from the differential pair is increased.The differential signaling version of the coupler therefore offers lowerfar-field electromagnetic radiation levels than a single endedimplementation. In addition to this differential embodiment, the couplermay be used in a single ended implementation, where a single conductorcouples electromagnetically to a single conductor, as shown in FIG. 6A.

[0038] In addition, the effects of far-field radiation may be furtherreduced by selecting an even number of conductor segments (e.g., eightsegments) for the coupler. Thus offers potentially lower far fieldelectromagnetic radiation levels compared to an implementation using anodd number of conductor segments.

[0039] The structure of FIG. 10A, which couples the differentialsignals, has a differential pair of conductors that alternately approacheach other and then turn away. Because the conductors 1014 and 1016 ofthe second transmission structure have segments with equal and oppositeangular displacements to conductors 1010 and 1012, respectively, thisstructure reduces the effects of capacitive crosstalk between conductors1010 and 1016 and conductors 1012 and 1014 due to misalignment from X,Yvariation of the conductors.

[0040]FIG. 10B shows an alternative geometry to the embodiment of FIG.10A. In FIG. 10B, the pair of differential conductors 1010 and 1012 havea segmented, angular rotated structure. Each segment of one conductorfrom the pair has an angular displacement such that the segment isparallel to a corresponding segment of the other conductor of the pairof conductors. This results in a differential pair where the conductorsmaintain parallel positions to each other throughout the length of thecoupler. In this configuration, the conductors 1014 and 1016 of thesecond transmission structure have segments with equal and oppositeangular displacements to conductors 1010 and 1012, respectively, whilealso keeping corresponding segments of conductors 1014 and 1016 parallelto each other. However, this alternative embodiment of FIG. 10B issubject to greater sensitivity to capacitive crosstalk than theembodiment of FIG. 10A.

[0041] For one embodiment, the coupler is designed to avoid impedancediscontinuities, or changes in the electromagnetic field structure, bynot using connections between multiple printed circuit board (PCB)layers, and avoiding abrupt (right angle) bends. (However, in analternative embodiment, a coupler may be designed with discontinuitiesor changes in field structure.) The discontinuity effects of the smallangular bends in between the coupler segments is further reduced bychamfering the outer edge of the bend slightly to keep the conductorwidth reasonably constant throughout the bend.

[0042]FIG. 11A represents electrical properties of an embodiment of asystem that includes multiple couplers in a digital bus communicationssystem. A conductor 1112, which may be on the motherboard of a computer,for example, incorporates two or more couplers 1140,1141 along itslength. The end 1112A of the conductor 1112 on the motherboard isconnected to a transceiver 1110 to permit the transmission or receptionof digital signals in a bidirectional manner. The end 1112B of theconductor 1112 on the motherboard is terminated with a resistor 1136equal to the impedance of the conductor.

[0043] The ends 1114B and 1134B of each coupled conductor are terminatedwith matching resistors 1130, 1132 for high frequency operation, theends 1114B and 1134B are selected to be the ends furthest from themotherboard transceiver 1110, because of signal directionality. Eachdaughter card has a transceiver 1120, 1122 connected to the end of thecoupled conductor 1114A, 1134A, respectively. The transceiver 1110transmits digital data which is received via the couplers 1140, 1141 bythe daughter card transceivers 1120, 1122. Conversely, transceivers1120, 1122 may separately transmit data through couplers 1140, 1141 forreception and decoding at transceiver 1110. FIG. 11B shows adifferential version of the multiple couplers for a bus communicationsystem.

[0044] This embodiment includes a data channel, such as a bus 1112,having substantially uniform electrical properties for transferringsignals among devices that are coupled through the data channel. Theuniform electrical properties are supported by an electromagneticcoupling scheme that allows higher frequency signaling to be employedwithout significantly increasing noise attributable to transmission lineeffects. This is achieved by ensuring that only a small amount of energy(e.g., less than 1%) is transferred between the bus and the coupleddaughter card. A preferred embodiment of this system is constructed insuch a way that daughter cards containing devices 1120 and 1122 may beremoved from or inserted to the system with little effect on thecommunication bandwidth of the bus.

[0045]FIG. 12A shows an embodiment of a cross-section of the coupler ofFIG. 10A, shown at the point where the conductors cross. A differentialpair of conductive signal traces 1230A and 1230B are coupled withanother differential pair of conductive signal traces, 1236A and 1236B.Dielectric 1212 separates conductive signal traces 1230A and 1230B.Dielectric 1220 separates conductive signal traces 1236A and 1236B.Dielectric 1216 separates the differential pairs. Conductive referenceplanes 1210 and 1222 provide return paths for the conductive signaltraces. The coupler may be constructed as an integral part of thecomputer motherboard. The conductive components 1230A, 1230B, 1236A,1236B of the coupler with selected width (e.g., 5 mils) and thickness(e.g., 1.4 mils) may be constructed using conventional etchingtechniques on the surface of a dielectric sheet 1216. The sheet 1216 mayhave a preferred thickness (e.g., 3.5 mils) and dielectric constant(e.g., 4.5). Additional dielectric layers 1212 and 1220, with preferredthickness (e.g., 12 mils) and dielectric constant are added to providethe required spacing between the coupler elements 1230A, 1230B, 1236A,1236B and the outer conductive reference planes 1210, 1222. The endconnections to the motherboard coupled conductors can then be connectedto the daughter card using conventional impedance controlled electricalconnectors as is currently common practice.

[0046] By placing cross-coupled conductors of the coupler between upperand lower conductive reference planes, 1210 and 1222, as shown in FIG.12A, a dual stripline structure is formed. Stripline structures have thesame even mode propagation velocity (the velocity for the wavepropagation mode between the conductors and the reference planes) as theodd mode propagation velocity (the velocity of the wave propagation modebetween the individual conductors of the coupler). This results inbroadband behavior, allowing the coupler to operate up to frequencies inthe microwave region.

[0047] Alternatively, the coupler may include a microstrip referenceplane, a coplanar reference plane, or may have no reference plane atall. One alternative embodiment is shown in FIG. 12B, which shows thetwo pairs of conductors 1230 and 1236 separated in a dielectric mediumwith no reference planes. This structure will form an EM coupler,however, it is not particularly suited for impedance control or widebandwidth characteristics.

[0048]FIG. 12C shows a microstrip configuration for the coupler withboth pairs of conductors 1230A, 1230B, and 1236A, 1236B referenced to asingle reference plane 1222. This microstrip embodiment improves theimpedance and bandwidth characteristics over that of FIG. 12B.Alternatively, a coplanar waveguide structure of FIG. 12D may beconstructed with reference conductors 1210 and 1222 in the same plane asthe corresponding conductive signal lines 1230A, 1230B and 1236A, 1236B.

[0049] The dielectrics in FIGS. 12A through 12D may be any dielectricmaterial, for example air or FR4. The bandwidth may be improved byselecting dielectric materials with similar dielectric constants. InFIGS. 12A through 12D, conductors 1230A and 1230B may have a differentwidth than conductors 1236A and 1236B. Also, dielectric 1212 may have adifferent thickness than dielectric 1220.

[0050] A separable embodiment of the coupler of FIG. 10A is exemplifiedin the cross-sectional view of FIG. 13. In this embodiment, motherboardconductors 1336A and 1336B are constructed on the outer layers 1360 of aprinted circuit card, with a width such as 8 mils for example, and athickness of 2.1 mils for example. The daughter-board conductors 1330Aand 1330B are contained in a flexible circuit 1350, which is pressedonto the surface of the motherboard. The conductors 1330A and 1330B maybe 10 mils wide and 0.7 mils thick, for example. In FIG. 13, conductivereference plane 1322 is an internal power or ground plane as commonlyused in printed circuit motherboards. The dielectric layer 1320 withpreferred thickness and dielectric constant (e.g., 5 mils and 4.5,respectively) is used to provide the correct spacing between themotherboard conductive signal traces 1336A, 1336B and the conductivereference plane 1322.

[0051] The outer surface of the board may be coated with a thindielectric coating or solder mask 1318, although this is not essentialto the operation of the coupler. The daughter card portion of thecoupler is provided with a conductive reference plane 1310 attached tothe top surface of a flexible dielectric 1312 with preferred thickness(e.g., 2 mils) and dielectric constant (e.g., 4.5). The daughter cardconductive signal traces 1330A, 1330B are constructed on the lowersurface of the flexible dielectric 1312. A dielectric adhesive 1314 isused to attach a dielectric or cover-lay film 1316 with preferredthickness (e.g., 0.5 mils) and dielectric constant (e.g., 3.8). Therequired coupling coefficient is achieved by selecting the preferredthicknesses and dielectric constants for the dielectric 1316 when takinginto the account the expected manufacturing variations in the dielectriccoating 1318 and airgaps 1340 in addition to other variations in thecoupler geometry and materials.

[0052] Although FIG. 13 shows a dual stripline embodiment, alternativessuch as a microstrip embodiment, a coplanar embodiment, or an embodimentwithout a reference plane may be used, as discussed above. Furthermore,conductors 1330A and 1330B may be a different width than conductors1336A and 1336B. Also, dielectric 1312 may be different thickness thandielectric 1320.

[0053]FIG. 14 shows a view in the plane orthogonal to that of FIG. 13.The flexible circuit 1350 for daughter card 1355 is folded into acircular loop, with the longitudinal axis of the signal conductors 1330Aand 1330B lying along the loop circumference. The ends of the conductivesignal traces 1330A and 1330B are connected to conductive etches on thetwo outer faces of the daughter card 1355 in order to provide connectionto the transceiver and terminating resistors mounted on the daughtercard 1355.

[0054] The loop is then pressed onto the top surface of the motherboard1365 so that the longitudinal axes of each motherboard conductor 1336Aand 1336B is parallel with, and in the desired proximity to, thecorresponding coupled flex circuit conductor. The length of the flexiblecircuit and vertical position of the daughter card are adjusted bymechanical means such that the motherboard conductors are in the desiredproximity to the flex circuit conductors for a length L, which isselected to ensure that the capacitive and inductive couplingcoefficients fall within the desired range of values. The length L maybe 1 cm for example.

[0055] Some bandwidth reduction may be present in the flex stripimplementation of FIG. 14 if the flex strip is made of polyimide(dielectric constant=3.8) and the motherboard is made of FR4 glass-epoxy(dielectric constant=4.5). These materials are commercially availablefrom well-known vendors such as 3M or DuPont. This may be eliminated ifthe FR4 is replaced with a material with a dielectric constant equal orclose to that of polyimide, like Rogers RO4003 or similar lowerdielectric constant materials. Rogers RO4003 is available from theRogers Corporation. In the embodiment where the coupler is buried in themotherboard, the bandwidth may be limited by the dielectric losses inthe FR4 material used in low-cost PCB assemblies. Again, the use ofmaterials with lower dielectric losses like Rogers RO4003 relieves theselimits.

[0056]FIG. 15 shows a detail of the contact area between the flexiblecircuit and the top surface of the motherboard corresponding to theembodiment outlined in FIGS. 13 and 14. Arranging the motherboardconductors 1336A, 1336B, in selected proximity to the flex circuitconductors 1330A, 1330B, creates the coupler. The motherboard-connectedsegments are lying in a plane where adjacent segments are arranged withan alternating angular displacement about the longitudinal axis of theconductor. The flex circuit conductors, similarly segmented, arearranged so that the angular displacements of its segments are inopposite sense to the corresponding segments in the motherboard. Thecomposite structure may thus have the zig-zag geometry as shown in FIG.6A.

[0057] These and other embodiments of the present invention may berealized in accordance with these teachings and it should be evidentthat various modifications and changes may be made in these teachingswithout departing from the broader spirit and scope of the invention.The specification and drawings are, accordingly, to be regarded in anillustrative rather than restrictive sense and the invention measuredonly in terms of the claims.

What is claimed is:
 1. An apparatus comprising: a circuit board; and aconductive trace on the circuit board, the conductive trace including atransmission structure having a plurality of transmission sections, eachtransmission section having an angle of deflection relative to an axisparallel to the transmission structure, such that the angle ofdeflection of each transmission section enhances electromagneticcoupling of the transmission structure.
 2. The apparatus of claim 1,wherein the transmission structure comprises a single conductor.
 3. Theapparatus of claim 1, wherein the transmission structure comprises atleast one differential pair of conductors.
 4. The apparatus of claim 3,wherein the transmission sections form a zigzag geometry.
 5. Theapparatus of claim 1, wherein the transmission sections form a zigzaggeometry.
 6. The apparatus of claim 1, further comprising: a planarconductive reference plane that is parallel to a plane containing thetransmission structure; said planar conductive reference plane providinga reference potential to the transmission structure.
 7. The apparatus ofclaim 1, wherein the transmission structure can transfer signals in anelectronic system.
 8. The apparatus of claim 7, wherein the electronicsystem is selected from the group comprising: a computer system, acomputer bus, a computer motherboard, a daughter card, a multi-chipmodule, an integrated circuit, a flex circuit, a printed circuit board,and a cable circuit.
 9. An apparatus comprising: a circuit board; aconductive trace on the circuit board; and a first transmissionstructure having a selected geometry and connected to the conductivetrace, such that said transmission structure is capable of forming anelectromagnetic couple with a second transmission structure when saidfirst and second transmission structures are proximate to each other,said electromagnetic couple having at least three fringe capacitanceregions.
 10. The apparatus of claim 9, wherein the electromagneticcouple has at least two parallel plate capacitance regions
 11. Theapparatus of claim 9, wherein the selected geometry is a zig-zaggeometry.
 12. The apparatus of claim 9, wherein the first transmissionstructure comprises at least one differential pair of conductors. 13.The apparatus of claim 12, wherein the selected geometry is a zig-zaggeometry.
 14. An apparatus comprising: a circuit board; conductive meansfor conducting a signal, said conductive means on the circuit board; anda first transmission means for receiving the signal from said conductivemeans and for transmitting said signal, said first transmission meanshaving a selected geometry, such that said first transmission means iscapable of forming an electromagnetic couple with a second transmissionmeans when said first and second transmission means are proximate toeach other, said electromagnetic couple having a plurality of parallelplate capacitance regions and three or more fringe capacitance regions.15. The apparatus of claim 14, wherein the selected geometry is azig-zag geometry.
 16. The apparatus of claim 14, wherein said firsttransmission means comprises at least one differential pair ofconductors.
 17. The apparatus of claim 14, wherein the selected geometryis a zig-zag geometry.